1. Field of the Invention
The present invention relates to a network relay device, and more specifically to detecting a failure occurring in a network, which a network relay device is connected with.
2. Description of the Related Art
In the Ethernet (registered trademark)-based layer 2 network, Spanning Tree protocol (STP) according to the IEEE 802.1d standard has been known as the protocol to make the switches redundant. The STP causes control frames called BPDU (Bridge Protocol Data Unit) to be exchanged between the switches, so as to logically eliminate the loop structure of routing and check the status of a network, which the switches are connected to, such as the occurrence or non-occurrence of a failure. Ring protocol has also been known as the protocol to make the switches redundant. The Ring protocol causes control frames called Health check frames to be exchanged between the switches, so as to logically eliminate the loop structure of routing and check the status of a network, which the switches are connected to.
Link Aggregation according to the IEEE802.3ad/IEEE802.1AX standard, on the other hand, has been known as the configuration to make the physical lines redundant. The Link Aggregation is capable of bundling or aggregating multiple physical lines and treating the bundled physical lines as one virtual line. The Link Aggregation advantageously expands the bandwidth, since the bandwidth of the virtual line is the sum of the bandwidths of the bundled physical lines. Even on the occurrence of a failure in part of the bundled physical lines, the Link Aggregation advantageously enables communication to be continued by using the remaining part of the bundled physical lines.
Recently, networks employing the technique of making the switches redundant in combination with the technique of making the physical lines redundant have increased in number to enable the high-speed, high-capacity and highly-reliable communication, in core networks and metro networks as well as access networks.
In the network employing the technique of making the switches redundant in combination with the technique of making the physical lines redundant, for example, in the network employing the Ring protocol in combination with the Link Aggregation, Health check frames are sorted into and sent by the respective physical lines included in the virtual line according to a predetermined sorting method. Periodical transmission of Health check frames may be concentrated on one specific physical line included in the virtual line. On the occurrence of a failure in this specific physical line, the occurrence of a failure in the whole Ring network may be falsely detected, although the failure does not actually occur over the whole virtual line (i.e., no failure occurs in the other physical lines included in the same virtual line).
This problem is not characteristic of the concrete protocols or their combinations, such as STP, Ring protocol or Link Aggregation but is commonly found in any network relay device configured to make physical lines redundant and exchange control frames (or packets) for use in checking the status of the network with another network relay device.